Sawtooth line circuit for a cathode ray tube

ABSTRACT

A circuit arrangement is described for generating a sawtooth current in a cathode ray tube deflection coil. The circuit arrangement has a first and a second multiresonant section. The sections are arranged in series. The cathode ray tube deflection coil is part of the first section. The second section has an inductor. Each section provides a trace period in which the sawtooth current is generated in the cathode ray tube deflection coil and a retrace period in which flyback of the sawtooth current is provided. The circuit arrangement further has a current control circuit coupled to the inductor for controlling the course of the sawtooth current.

[0001] The present invention relates to a circuit arrangement forgenerating a sawtooth current in a cathode ray tube deflection coil.

[0002] The present invention also relates to a display device, such as amonitor or a television, comprising a cathode ray tube, and to atelevision set comprising such a display device.

[0003] Such a circuit arrangement, display device, and televisionapparatus are known from U.S. Pat. No. 3,906,305. The known circuitarrangement includes a first and a second multiresonant section arrangedin series. Each multiresonant section comprises a trace capacitor, aretrace capacitor, and a coil. One of the coils in the two or moresections is a line deflection coil of a cathode ray tube (CRT). Thesecond section comprises an inductor in the form of a coil.Simultaneously, each section provides a common trace or first resonantperiod, in which the sawtooth current is generated in the CRT deflectioncoil and a similar current is generated in the coil-type inductor. Thetrace period is followed by a retrace period in each of the sections, inwhich the currents go back to their original value in a very shortsecond resonant period of time. Semiconductor blocking means areconnected in parallel to each of the sections and to the seriesarrangement of sections to control these trace and retrace periods. Thecircuit arrangement further has a driver modulator that is coupled inparallel to the trace capacitor of the second multiresonant section inorder to provide the well known east-west raster correction of the CRTline deflection current. The inventor of the present invention has foundthat it is a disadvantage of the known circuit arrangement that itcomprises many components which, apart from being relatively expensiveand space consuming, also show a considerable power dissipation, whichin general makes them more susceptible to breakdown.

[0004] Therefore it is an object of the present invention to provide aless dissipative, but more reliable and compact circuit arrangementcomprising a smaller number of vulnerable components.

[0005] According to the invention, a circuit arrangement is for thispurpose provided for generating a sawtooth current in a cathode ray tubedeflection coil, the circuit arrangement comprising a first and a secondmultiresonant section arranged in series, the cathode ray tubedeflection coil being comprised in the first section, and the secondsection comprising an inductor, each section providing a common traceperiod and a common retrace period to provide a flyback of said sawtoothcurrent, and the circuit arrangement comprising a current controlcircuit coupled to the inductor for controlling the course of thesawtooth current. In this respect, the course of the sawtooth current isits instantaneous value as a function of time. It is an advantage of thecircuit arrangement according to the present invention that, althoughthe known arrangement has been scrutinized since its first developmentmany years ago, the current control now devised makes the voluminous andexpensive trace capacitor in the second multi-resonant sectionsuperfluous. However, also the second multiresonant section remainscapable of acting properly in a kind of multi-resonant mode, both duringthe trace and during the retrace period.

[0006] In addition, the component count of the circuit arrangementaccording to the invention, which has never been essentially reducedsince its original design, has now been reduced in a way which evenallows complete line power stage integration on an integrated circuit.This brings one-chip television more easily within reach.

[0007] In an embodiment of the circuit arrangement according to theinvention, the current control circuit is a current switching circuitwhich may be coupled between two supply voltages of the circuitarrangement.

[0008] It is an advantage of the circuit arrangement according to theinvention that the current control circuit can be embodied by a simplecurrent switching circuit. Such a switching circuit may be coupled toany of the power supply voltage terminals that are already present inpresent-day display or image devices, in practice the supply voltage orground terminal, respectively. In that case the presence of additionalpower supply voltage rails is not required.

[0009] In a further embodiment of the circuit arrangement according tothe invention, the circuit arrangement is arranged to effect a CRTeast-west correction, in a simple embodiment by dynamically adjusting acurrent switching point in time marking a start of the trace period.

[0010] East-west correction to avoid barrel-shaped image distortion canthen be effected by shifting the current switching point in time inorder to influence the maximum and minimum value of the line current tothe line deflection coil around the neck of a CRT in a display device,such as a monitor or television.

[0011] In an embodiment of the circuit arrangement according to theinvention that is easy to implement, the inductor is a coil or a gyratorcircuit.

[0012] The circuit arrangement according to the invention will now beelucidated further together with its additional advantages, whilereference is being made to the appended drawing. In the drawing:

[0013]FIG. 1 shows a basic circuit diagram of an embodiment of a circuitarrangement according to the present invention; and

[0014]FIG. 2 shows various voltage and current, timing and waveformgraphs arising in the circuit arrangement according to FIG. 1.

[0015]FIG. 1 shows a basic circuit diagram of a circuit arrangement 1.Such a circuit arrangement 1 forms part of a circuit for generating aline current in a line deflection coil L1 in a display device such as,for example, a monitor or a television. The arrangement 1 also comprisesa line transformer only partly shown in FIG. 1 and referenced L3.Further details, as regards additionally effecting image corrections orthe deriving of an extremely high voltage (EHT) by the line transformerfrom a line voltage at terminal A for the acceleration anode of a colorCRT, are outlined in U.S. Pat. No. 3,906,305, whose disclosure isincluded herein by reference.

[0016] The circuit arrangement 1 includes a first and a secondmultiresonant section I and II, respectively. Starting from a fixed DCvoltage at supply terminal Vb, line transformer coil L3, and sections Iand II are arranged in series and coupled to ground in the basic circuitdiagram of FIG. 1. Section I comprises a series arrangement ofdeflection coil L1 and a trace capacitor Ct, which series arrangement isconnected in parallel to a retrace capacitor Crt1 and a semiconductortransition, in particular a diode D1 with an indicated conductivitydirection. Section II comprises a series arrangement of an inductor,which may be a gyrator circuit for simulating an inductance or a coilL2, and a current control circuit 2, which series arrangement of sectionII is connected in parallel to a retrace capacitor Crt2 and asemiconductor transition, in particular a diode D2 with an indicatedconductivity direction, identical to that of diode D1. Apart from diodesD1 and D2, a controllable semiconductor means in the form of a drivabletransistor T1 forms blocking means whose function will be explainedbelow.

[0017] The operation of the circuit arrangement 1 of FIG. 1, which willbe explained with the help of the voltage and current, timing andwaveform graphs of FIG. 2, is as follows. Diodes D1 and D2 conductduring part of a so-called trace period Tr, which is common to bothmultiresonant sections I and II. Consequently a voltage across capacitorCt in multiresonant section I is applied to line deflection coil L1during a first resonance period. Since said voltage and the conductanceL1 are constant, the differential ratio dIL1/dt will also be constant asa function of time so as to form a sawtooth part of the deflectioncurrent. This means that a line will be drawn on the CRT from left toright. Before current IL1 shown in FIG. 2 crosses zero approximatelyhalf-way of the trace time Tr, transistor T1 receives a drive signalHdrive rendering T1 conducting and allowing current IL1 to becomepositive. Multiresonant section II advantageously lacks such avoluminous and expensive trace capacitor, so in a way the firstresonance period thereof is simulated. That is, current IL2 throughinductor L2 is controlled by current control circuit 2, which maycontain two controllable current sources. In FIG. 2 these controllablecurrent sources are simplified by the current switching circuit 2 thencomprising controllable switches S1 and S2 coupled between two supplyvoltages, here Vcc and ground, of the circuit arrangement 1. During apart of the trace period Tr, wherein controllable switch S1 is closedand controllable switch S2 is open, dI12/dt is kept constant by anappropriate control of the current circuit 2. Due to the terminal Vbhaving a fixed dc voltage, any change in one of the sections I and IIwill be compensated for by a similar negative change in the other one ofthe sections I and II. If current Il1 is larger than current Il2, asurplus current will flow through transistor T1 and diode D2, whileconversely if Il2>Il1 a surplus current will flow through T1 and D1. Soany change in controlled current Il2 will lead to a subsequent change ofcurrent in Ill. A CRT east-west correction can now be adjusteddynamically by influencing current switching points in time marked P1and P2. In addition the current control circuit 2 lacks a largedissipative control transistor including a heat sink, so that a higherdegree of integration becomes possible. The switching points P1, P2 markthe adjustable starts of the trace period part after which dIl1/dt anddIl2/dt are both constant. This is schematically shown in FIG. 2 by thevertical dashed lines, while the possibility of adjustments of theswitching points is indicated by an arrow to the left. The more theswitching point P1 or P2 is shifted to the left, the shorter the lengthof a line drawn on the CRT will be, thus allowing for an appropriateeast-west image or barrel correction. This is because the shifting tothe left of the switching point P1, P2 in FIG. 2 increases the amplitudeof the current Il2 in section II, as a result of which the currentamplitude of Il1 decreases, resulting in the shorter line length.

[0018] Now during a so called flyback or retrace period Rt—common toboth sections I and II—wherein S1 and S2 are timed such that S2 closesand S1 opens, T1 is controlled to open, and consequently retracecapacitors Crt1 and Crt2 are supplied with respective inductive currentsfrom L1 and L2, respectively, during a second resonance period. Thisleads to diode voltages across diodes D1 and D2 indicated as VD1 andVD1+D2. In practice the voltage at terminal A may approximate a desiredvalue of approximately 1200 V if a stable EHT voltage is to be derivedtherefrom by the line transformer. The time scale of the resonanceduring period Rt is much shorter than that of Tr, because the capacitorvalue of capacitor Ct is much larger than the capacitor value ofcapacitor Crt1. At the instant when the diode voltages become zeroagain, diodes D1 and D2 simultaneously become conducting, thus markingthe beginning of a next trace period Tr.

[0019] It is noted, that in this document the verb ‘comprise’ and itsconjugations do not exclude the presence of other elements or steps thanthose listed, that the word ‘a’ or ‘an’ preceding an element does notexclude the presence of a plurality of such elements, and that anyreference signs do not limit the scope of the claims. Furthermore, thescope of the invention is not limited to the embodiments, and theinvention lies in each and every novel feature or combination offeatures described above.

1. A circuit arrangement for generating a sawtooth current in a cathoderay tube deflection coil, the circuit arrangement comprising a first anda second multiresonant section arranged in series, the cathode ray tubedeflection coil being comprised in the first section, and the secondsection comprising an inductor, each section providing a common traceperiod and a common retrace period to provide a flyback of said sawtoothcurrent, and the circuit arrangement comprising a current controlcircuit coupled to the inductor for controlling the course of thesawtooth current.
 2. The circuit arrangement according to claim 1,wherein the current control circuit is a current switching circuit. 3.The circuit arrangement according to claim 2, wherein the currentcontrol circuit is coupled between two supply voltages of the circuitarrangement.
 4. The circuit arrangement according to claim 1, whereinthe current control circuit is arranged to effect a cathode ray tubeeast-west correction.
 5. The circuit arrangement according to claim 4,wherein the cathode ray tube east-west correction is effected bydynamically adjusting a current switching point in time marking a startof the trace period.
 6. The circuit arrangement according to claim 1,wherein the inductor is a coil or a gyrator circuit.
 7. A display devicecomprising a cathode ray tube, the display device comprising a circuitarrangement for generating a sawtooth current in a cathode ray tubedeflection coil coupled to the cathode ray tube, the circuit arrangementcomprising a first and a second multiresonant section arranged inseries, the cathode ray tube deflection coil being comprised in thefirst section, and the second section comprising an inductor, eachsection providing a common trace period and a common retrace period toprovide a flyback of said sawtooth current, and the circuit arrangementcomprising a current control circuit coupled to the inductor forcontrolling the course of the sawtooth current.
 8. A television setcomprising a display device according to claim 7.